1. Field of the Invention
This invention relates to a micrometer for measuring dimensions of a workpiece to be measured such as a length, thickness and the like from a displacement of a spindle.
2. Description of the Prior Art
Heretofore, the commonest micrometer has been a so-called screw feed type micrometer in which a portion of an inner sleeve affixed to a main body is internally threaded with high accuracies, a portion of a spindle externally threaded with high accuracies is threadably coupled to the internally threaded portion, and the spindle is rotated by a thimble integrally affixed to the spindle, to thereby carry out the measurement of a workpiece to be measured. The screw feed type micrometer of the type described is advantageous in that the micrometer has high dust-excluding characteristics because the inner construction including threaded portions of the micrometer is substantially fully closed, and the clamped state of the workpiece can be secured due to the self-locking action of the threaded portions. However, since the spindle as a whole is integrally rotated by the rotation of the thimble, the forward end of the spindle rotates during measuring operation. Thus, when a material high in flexibility such as a non-rigid plastics plate is measured, wrinkles and the like are caused to the workpiece, thereby proving to be unsuitable for the measurement of the aforesaid material. In the micrometer of the type described, when the materials not only the flexible material described above but also other materials are measured, the forward end portion of the rotating spindle comes into contact with the workpiece, whereby wear is caused to the workpiece as well as the spindle a torsional force acting on the workpiece is generated from the spindle, so that the micrometer has not been desirable for maintaining the measurement with high accuracies.
Now, there have been proposed so-called linear type micrometers having various constructions, wherein even if the thimble is rotated, the spindle is displaced without rotation. However, the micrometer of the type described are so complicated in construction that, for example, an intermediate cylindrical member is provided between the spindle and the thimble, further, frictionally engaging means and spring biasing means are provided and so on. Particularly, the fact that such a complicated mechanism must be incorporated in the thimble results in an increased diameter of the thimble, so that the complicated construction has not been suitable for the micrometer generally being compact in size and requiring one-hand operation.